Signal transducer and activator of transcription (STAT) proteins are transcription factors which transduce signals from various extracellular cytokines and growth factors to a nucleus. Seven (7) subtypes of STAT proteins (i.e., STAT1, STAT2, STAT3, STAT4, STAT5a, STAT5b and STAT6) are currently known, and generally they consist of about 750-850 amino acids. In addition, each subtype of STAT proteins contains several conserved domains which play an important role in exhibiting the function of STAT proteins. Specifically, five (5) domains from N-terminus to C-terminus of STAT proteins have been reported including coiled-coiled domain, DNA binding domain, linker domain, SH2 domain and transactivation domain (TAD)). Further, X-ray crystalline structures of STAT1, STAT3, STAT4 and STAT5 have been reported since 1998 (Becker S et al., Nature, 1998, 394; Vinkemeier U et al., Science, 1998, 279; Chen X et al., Cell, 1998, 93; D. Neculai et al., J. Biol. Chem., 2005, 280). In general, receptors to which cytokines and growth factors bind are categorized into Class I and Class II. IL-2, IL-3, IL-5, IL-6, IL-12, G-CSF, GM-CSF, LIF, thrombopoietin, etc., bind to Class I receptors, while INF-α, INF-γ, IL-10, etc., bind to Class II receptors (Schindler C et al., Annu. Rev. Biochem., 1995, 64; Novick D et al., Cell, 1994, 77; Ho AS et al., Proc. Natl. Acad. Sci., 1993, 90). Among them, the cytokine receptors involved in the activation of STAT proteins can be classified depending on their structural forms of extracellular domains into a gp-130 family, an IL-2 family, a growth factor family, an interferon family and a receptor tyrosine kinase family. Interleukin-6 family cytokines are representative multifunctional cytokines which mediate various physiological activities. When interleukin-6 cytokine binds to IL-6 receptor, it attracts gp-130 receptor to form an IL-6-gp-130 receptor complex. At the same time, JAK kinases (JAK1, JAK2, JAK3 and Tyk2) in the cytoplasm are recruited to a cytoplasmic region of gp130 to be phosphorylated and activated. Subsequently, latent cytoplasmic STAT proteins are attracted to a receptor, phosphorylated by JAK kinases and activated. Tyrosine-705 near the SH2 domain located in the C-terminus of STAT proteins is phosphorylated, and the activated tyrosine-705 of each STAT protein monomer binds to the SH2 domain of another monomer in a reciprocal manner, thereby forming a homo- or heterodimer. The dimers are translocalized into a nucleus and bind to a specific DNA binding promoter to promote the transcription. Through its transcription process, various proteins (Myc, Cyclin D1/D2, Bcl-xL, Mcl, survivin, VEGF, HIF-1, immune suppressors, etc.) associated with cell proliferation, survival, angiogenesis and immune evasion are produced (Stark et al., Annu. Rev. Biochem., 1997, 67; Levy et al., Nat. Rev. Mol. Cell Biol., 2002, 3).
In particular, STAT3 protein is known to play a crucial role in the acute inflammatory response and the signal transduction pathway of IL-6 and EGF (Akira et al., Cell, 1994, 76; Zhong et al., Science, 1994, 264). According to the recent clinical report, STAT3 protein is constantly activated in patients with solid cancers occurring in prostate, stomach, breast, lung, pancreas, kidney, uterine, ovary, head and neck, etc., and also in patients with blood cancer such as acute and chronic leukemia, multiple myeloma, etc. Further, it has been reported that the survival rate of a patient group with activated STAT3 is remarkably lower than that of a patient group with inactivated STAT3 (Masuda et al., Cancer Res., 2002, 62; Benekli et al., Blood, 2002, 99; Yuichi et al., Int. J. Oncology, 2007, 30). Meanwhile, STAT3 was identified to be an essential factor for the growth and maintenance of murine embryonic stem cells in a study employing a STAT3 knockout mouse model. Also, a study with a tissue-specific STAT3-deficient mouse model, reveals that STAT3 plays an important role in cell growth, apoptosis, and cell motility in a tissue-specific manner (Akira et al., Oncogene 2000, 19). Moreover, since apoptosis by anti-sensing STAT3 was observed in various cancer cell lines, STAT3 is considered as a promising new anticancer target. STAT3 is also considered as a potential target in the treatment of patients with diabetes, immune-related diseases, hepatitis C, macular degeneration, human papillomavirus infection, non-Hodgkin's lymphoma, tuberculosis, etc. In contrast, although STAT1 has identical intracellular response pathways of cytokines and growth factors to those of STAT3, STAT1 increases inflammation and congenital and acquired immunities to inhibit the proliferation of cancer cells or cause pro-apoptotic responses, unlike STAT3 (Valeria Poli et al., Review, Landes Bioscience, 2009).
In order to develop STAT3 inhibitors, the following methods can be considered: i) inhibition of the phosphorylation of STAT3 protein by IL-6/gp-130/JAK kinase, ii) inhibition of the dimerization of activated STAT3 protein, and iii) inhibition of the binding of STAT3 dimer to nuclear DNA. Small molecular STAT3 inhibitors are currently under development. Specifically, OPB-31121 and OPB-51602 are under clinical studies on patients with solid cancers or blood cancers by Otsuka Pharmaceutical Co., Ltd. Further, S3I-201 (Siddiquee et al., Proc. Natl. Acad. Sci., 2007, 104), S3I-M2001 (Siddiquee et al., Chem. Biol., 2007, 2), LLL-12 (Lin et al., Neoplasia, 2010, 12), Stattic (Schust et al., Chem. Biol. 2006, 13), STA-21 (Song et al., Proc. Natl. Acad. Sci., 2005, 102), SF-1-066 (Zhang et al., Biochem. Pharm., 2010, 79) and STX-0119 (Matsuno et al., ACS Med. Chem. Lett., 2010, 1), etc. have been tested in a cancer cell growth inhibition experiment and in animal model (in vivo Xenograft model). Furthermore, although peptide compounds mimicking the adjacent amino acid sequence of pY-705 (STAT3) which binds to SH2 domain or the amino acid sequence of gp-130 receptor that JAK kinases bind were studied (Coleman et al., J. Med. Chem., 2005, 48), the development of the peptide compounds has not been successful due to the problems such as solubility and membrane permeability.